Temperature compensated crystal oscillator

ABSTRACT

A crystal controlled oscillator is frequency sensitive to variations in crystal load capacitance in circuit with the crystal, to variations in temperature and to operation over extended periods of time. A variable capacitor is provided in circuit with the crystal so as to correct for the crystal frequency drifts due to changes in operation of the crystal over extended periods of time. A separate fixed capacitor is connected in series with the crystal and in circuit with the variable capacitor to provide part of the frequency determining circuit of the crystal oscillator. A temperature compensation network is coupled across the fixed capacitor and is responsive to temperature changes to provide a correct degree of load capacitance change in the circuit so that the oscillator frequency is maintained within a given frequency tolerance regardless of the adjustment of the variable capacitor which is used to adjust the crystal for frequency drift.

United States Patent Mrozek I [54] TEMPERATURE COMPENSATED CRYSTALOSCILLATOR [52] US. Cl. ..331/116 R, 331/176 [51] Int. Cl. 03b 5/36 [58]Field ofSearch ..33l/1l6, 176, 66, 158, 159-164;

[56] References Cited UNITED STATES PATENTS 3,176,244 3/1965 Newell etal. .33 1/176 3,256,496 6/1966 Angel 3,322,981 5/1967 Brenig ..33l/l l6Feb. 8, 1972 FOREIGN PATENTS OR APPLICATIONS Primary ExaminerJohnKominski Attorney-Edward J. Norton [57] ABSTRACT A crystal controlledoscillator is frequency sensitive to variations in crystal loadcapacitance in circuit with the crystal, to variations in temperatureand to operation over extended periods of time. A variable capacitor isprovided in circuit with the crystal so as to correct for the crystalfrequency drifts due to changes in operation of the crystal overextended periods of time. A separate fixed capacitor is connected inseries with the crystal and in circuit with the variable capacitor toprovide part of the frequency determining circuit of the crystaloscillator. A temperature compensation network is coupled across thefixed capacitor and is responsive to temperature changes to provide acorrect degree of load capacitance change in the circuit so that theoscillator frequency is maintained within a given frequency toleranceregardless of the adjustment of the variable capacitor which is used toadjust the crystal for frequency drift.

2 Claims, 3 Drawing Figures TEMPERATURE COMPENSATED CRYSTAL OSCILLATORThis is a continuation of my copending application Ser. No. 591,860filed Nov. 3, 1966 and now abandoned.

This invention relates to oscillators and more particularly to animproved temperature compensated crystal oscillator.

Temperature compensated oscillators have been known for a number ofyears. This method of achieving an accurate and stable frequency sourceover wide temperature ranges has a number of important advantagescompared to the betterknown oven controlled oscillators. The temperaturecompensated oscillator has among other advantages (a the elimination ofwarmup time, (b) the reduction of power drain, and (c) improvement inlong term crystal stability because of the lower average operatingtemperature of the crystal. This type of circuit is particularlysuitable for use in portable and mobile applications where the powerdrain of an oven is intolerable, and a fast wannup time is desired. Alsothe temperature compensated crystal oscillator is particularly suitablefor use in applications where long term crystal frequency stabilizationis necessary.

The compensation for crystal frequency drifts due to temperature isusually accomplished in temperature compensated crystal oscillators byvarying the crystal load capacitance (C in a predetermined manner tocompensate for crystal frequency changes with temperature. Accuratecontrol of circuit components and crystal parameters is required toinsure that the crystal compensating network temperature characteristicmatches that of the crystal to the specified tolerance limits. Therequired load capacitance change AC, as a function of temperature can beprovided by a number of temperature sensitive networks such as athermistor capacitor or a thermistor voltage variable capacitor.However, because changes occur in the oscillator frequency over anextended period of time necessitating frequency adjustment, thecompensation after the frequency is adjusted may change which adds tothe overall frequency tolerance of the oscillator. Because of thiseffect, critical requirements are normally placed on the crystal itselfin terms of better aging and tighter tolerances.

It is an object of the applicant's invention to provide an improvedtemperature compensated crystal oscillator.

It is another object to provide an improved temperature compensatedcrystal oscillator in which changes in the compensation after theoscillator frequency is adjusted are minimized by minimizing thevariations of crystal frequency sensitivity to load capacitance.

It is a further object of the present invention to provide an improvedtemperature compensated crystal oscillator in which variations ofcrystal frequency sensitivity to load capacitance are minimized bymaking use of the resistive changes as well as the capacitive changes ofa thermistor capacitor network, or equivalent, with temperature.

Briefly, there is provided in accordance with one embodiment of theinvention a temperature compensated crystal oscillator having afrequency determining circuit comprising a fixed capacitance connectedin series with the crystal and a variable capacitor. The oscillator isfrequency sensitive to changes in the crystal load capacitance due tochanges in temperature. A temperature compensating network operates toalter the crystal load capacitance in a manner to compensate forfrequency drift with temperature within a given tolerance. The variablecapacitor can be operated to correct for long term crystal frequencydrift and, when so operated, can alter the degree of compensating loadcapacitance change affected by the temperature compensating network,whereby the range of tolerable frequencies is outside the giventolerance.

In accordance with the present invention, the temperature compensationnetwork is connected across only the fixed capacitance of the frequencydetermining circuit of the crystal oscillator. Anyaltering of the degreeof compensating load capacitance change by the temperature compensatingnetwork due to a frequency adjustment by the variable capacitor isminimized, thereby maintaining said given frequency tolerance.

FIG. 1 is a circuit diagram of one embodiment of the present invention;

FIG. 2 is a series of curves useful in describing the operation of theembodiment shown in FIG. 1; and

FIG. 3 is a circuit diagram of a temperature-compensated oscillatoraccording to a second embodiment of the applicants invention.

Referring to FIG. 1 of the drawing, an oscillator similar to theColpitts type embodying the present invention is shown. A transistor 10is shown illustratively as an NPN junction transistor and is biased by astabilized voltage applied at terminal 11. The positive terminal of aunidirectional potential source (not shown) is connected to terminal 11with its return terminal connected to conductor 12 at ground or otherreference potential. The emitter 15 of transistor-l0 is forward biasedwith respect to the base 13 by means of a resistor 16 connected betweenthe emitter l5 and ground. A pair of resistors l7 and 18 are connectedin series between the positive terminal 11 and ground. A connection fromthe junction of the resistors l7, 18 to the base 13 providesconventional transistor base bias. A resistor 20 and an RF bypasscapacitor 21 are connected in series between the positive terminal H andground with the junction of the capacitor 21' and resistor 20 connectedto the collector 14. An output coupling capacitor 22 is connected to theemitter 15. The frequency determining circuit comprises a crystal 25connected inseries with a fixed capacitance 27 and a variablecapacitance 26-between the base 13 and ground. The frequency determiningcircuit also includes a pair of fixed capacitors 28 and 29 seriesconnected between the base 13 and ground. A connection is completed fromthe emitter 15 to the junction of the capacitors 28 and 29. Capacitors28, 29 provide the correct amount of feedback to sustain oscillations.The total oscillator voltage appears across this frequency determiningcircuit which is in effect connected between the base 13 and collector14 of the transistor.

Solution of the voltage equivalent circuit of FIG. I in terms ofparallel emitter and base parameters is shown below:

7 W 7 7 (parts per million) (l) where f], crystal series resonantfrequency Af=fj,',, f= frequency of oscillations C, crystal motionalcapacitance C, crystal shunt capacitance C equivalent total parallelemitter-to, collector capacitance C capacitance series combination ofcapacitors 26 and 27 R equivalent total parallel emitter-to-collectorresistance including output resistance C equivalent total parallel baseto emitter capacitance R equivalent total base to emitter inputresistance Cs= effective total load capacitance given by i= i l C, C C IR, total circuit series resistance Frequency compensation isconventionally achieved by varying the crystal load capacitance (C,) tocompensate for the crystal frequency changes with temperature. Therequired load capacitance change (AC,) as a function of temperature, canbe provided by a number of temperature sensitive networks such as athermistor-capacitor or thermistor voltage variable capacitor. FIG. 1shows a compensation network 9 which may be for example a thermistorcapacitor temperature compensation network. For a given change intemperature, network 9 provides a given amount of compensatingcapacitance change AC and thermistor resistance change R,. Compensationnetworks can be connected in parallel with any of the circuit capacitorsor in parallel with the crystal. However, since C (capacitor 28) and C(capacitor 29) are large requiring large load capacitance changes (ACand/or AC for a given frequency change (AF), the more conventionalpractice is to place the small compensating capacitance AC which is partof and is controlled by the temperature sensitive network 9 in parallelwith a variable (trimmer) capacitor which is connected in series withthe crystal. This series variable (trimmer) capacitor is equivalent tothe capacitance C provided by the series combination of capacitors 26and 27 in FIG. I. From the equation (1) above, it is seen that thefrequency of oscillation depends also on circuit resistance (R,).However, the effect of resistance can be made negligible by making R C;and R C sufiiciently large. The value of the compensating capacitance(AC) is small compared to the load capacitance C, and therefore therelationship between compensating capacitance AC. and frequency changeAF can be obtained by differentiating equation l first with respect toC, and then with respect to the single variable capacitor yielding:

With Q small relative to unity, the usual case, the frequency change AFis inversely proportional to the square of the value of the capacitanceC, this relationship holds true for agiven compensating capacitance ACat any temperature. Therefore, in the conventional case where a variablecapacitor is used and both C; and C, are larger, the frequency change AFis inversely proportional to the square of the value of the variablecapacitance. When a given variable trimmer frequency range DF (which isequal to the difference between the highest frequency F l and the lowestfrequency F 2 by which the crystal frequency is tunable by the variabletrimmer capacitance) is required with a corresponding load capacitancechange DC (which is equal to the difference between the load capacitanceC at the high frequency F and the load capacitance C at the lowfrequency F the ratio of the frequency compensation at the extremes ofthe crystal frequency controlled by the variable capacitance is givenapproximately by:

C load capacitance corresponding to high frequency (F C load capacitancecorresponding to low frequency (F With a typical crystal having thevalues C 6 pf., C3, 24 pf, C, 0.03 pf. and trimmer frequency range DF='70 p.p.m., the compensation capacitance change will be 28 percentgiving a variation of :14 percent within the trimmer frequency range DF.

The meaning of this variation may be clearer if one considers a givencompensation AC of 14 p.p.m. required at a particular temperature ofinterest. After an extended period of time, adjustment of oscillatorfrequency by a trimmer capacitor may be required due generally tocrystal aging. When such trimmer capacitor adjustment is made, thecompensation AC could itself change by as much as $2.0 p.p.m. whichwould add to the overall frequency tolerance. FIG. 2 shows the variationin compensation in the commonly used and above mentioned variabletrimmer capacitor. Curve A shows the change in frequency per change intemperature without using a compensation network 9. Curves B, C and Dshow the change in frequency per change in temperature for the low-,highand middle trimmer frequencies to which the crystal is tunable bythe capacitor respectively using compensation network 9. It is clearthat with an overall frequency tolerance of 5 p.p.m. required, forexample, as shown in dotted lines, the oscillator frequency at thelow-trimming range B will for the example given be outside and below thetolerance limit.

In accordance with the applicant's present invention the effect of thedegree of compensation changing whenever a correction of the crystalfrequency is required is reduced by coupling the compensatingcapacitance eflectively in parallel with a fixed capacitor 27 andcoupling the compensating capacitance effectively in series with thetrimming capacitor 26. As shown in the circuit of FIG. I, the trimmercapacitor C described above is divided into two series components.Capacitor 26 (C,) is variable and used for frequency trimming. Capacitor27 (C is used for compensation and is fixed such The frequencycompensating network 9 (AC is placed in parallel with fixed capacitor 27(C rather than in parallel with the total variable capacitance C. Bydifferentiating equation (I) with respect to C the compensatingfrequency change AF is given by:

is small relative to unity, the frequency change is inverselyproportional to the square of the fixed capacitance 27 (C and thereforewill remain substantially constant within the trimming capacitor rangeDF. The amount of variation will depend only on the particular value ofQ relative to unity. If AF and AF are the frequency changes at theextremes of the crystal frequency controlled by the capacitance 26 inseries with fixed capacitor 27, the ratio in this case is givenapproximately by:

In accordance with another embodiment of the applicants presentinvention, the effects of the degree of compensation changing whenever acorrection of the crystal frequency is required is further reduced bymaking use of both the capacitive and resistive changes of athermistor-capacitive network, or equivalent circuit, where thecompensation process includes capacitance change and resistance changeexpressed as a function of temperature. In the case ofthermistorcapacitance compensation the two functions are mutuallydependent but it is possible to arrive at a circuit wherein thevariables can be independently controlled. FIG. 3 shows such a circuitwhich is a modification of the circuit shown in FIG. 1. A transistor 40is shown illustratively as an NPN transistor and biased by a stabilizedvoltage applied at tenninal 41. The positive terminal of aunidirectional potential source (not shown) is connected to terminal 41with its return terminal to ground or other reference potential.Resistors 30, 31 and 32 provide the conventional transistor bias butsince resistor 32 in this circuit also provides a load in parallel withthe variable capacitor 38, it is part of the compensation and the valuesare carefully selected. Capacitors 35, 36, 37 and 38 make up the crystalload capacitance. A resistor 42 and an RF bypass capacitor 43 areconnected in series between the positive terminal 41 and ground with thejunction of capacitor 43 and resistor 42 coupled to collector 50.Capacitor 44 is an output coupling 5 capacitor. The frequencydetermining circuit comprises crystal 47 in series with fixed capacitor35 and includes capacitor 37 and variable capacitor 38. Capacitors 37and 38 control the amount of feedback to sustain oscillations. Capacitor38 is made variable and is used for frequency trimming. Capacitor 35 (Cis a fixed capacitor across which a temperature sensitive networkcomprising capacitor 36 (AC!) and thermistor resistance (R,) 45 isconnected. The solution of the voltage equivalent circuit gives theapproximate frequency of oscillation as presented in the above equationl i=( i)( QR. 0.12.) fi 0+ s) I+CERE+CBRE (1) 1 1 1 1 where C, Is givenby E+E+FB C capacitor 35 (C;) capacitor 36 (AC,) at referencetemperature where thermistor resistance 45 (R,) is small,

AR,= small resistance in series with capacitor 36 and is added to thetotal resistance R,.

Assuming C R can be made much larger than C R the expression can berewritten as:

Examination of the Equation (6) above indicates that the frequency ofoscillation is made up of two parts, one dependent on C, and independentof R, and the other dependent on R, and almost independent of (1,, sinceQ controlled by ERE' The effect of the frequency change due tocapacitance change AF C and the frequency change due to resistancechange AF (R,) are used to achieve compensation independent of thetrimmer frequency capacitor 38. From Equation (6) frequency due to C,=F(C,)

- change of C, and R, change can be obtained by differentiating F(C,)with respect to C, and F(R,) with respect to R yielding: Frequencychange due to small A...

-C 10AC Ac,=AF c,

Since compensation is applied in parallel with fixed capacitor 35 (Q)frequency change due to small compensating capacitance change 2 2C}(14-5 Frequency change due to small C x 1 AR.=AF(R.)= 06 (in 1+5) CERNow, compensating capacitance change AC is negative (less capacitance),when AR, is positive (more resistance). Thus, when the temperaturechanges from the reference temperature to a lower temperature, bothchanges are positive, and therefore, the total frequency change is:

The following conditions can be observed from Equation l2 in consideringthe extremes of the crystal frequency controlled by the variablecapacitor 38 (1) at high-trimming frequency, both C and C, are small; sothat the first term of the Equation 12) is small and the second term ofthe equation is large. (2) at low-trimming frequency, both C and C. arelarge; so that the first term of the Equation (12) is large and thesecond term is small. Therefore, within a given variable trimmercapacitance range DF, the change in amount of frequency compensation dueto the capacitive effect is counteracted by the opposite change incompensation due to resistive effect.

In the embodiment shown in FIG. 3 conditions for perfect cancellation ofthese two changes can be obtained by differentiating Equation ([2) withrespect to C and equating to zero, which yields:

Thus, the required resistance 32 (R is given in terms of AR,, AC, C Cand the crystal parameter. Since 2 is very small compared to unity,resistance 32 (R is practically independent of C therefore, an almostperfect stability of compensation is achieved within the trimmercapacitor range.

in practice, the resistance change AR, of a simple thermistor-capacitorcompensating network is dependent on the compensation capacitance changeAC. Thus, when an exact change in frequency AF is required according todesign requirements, Equation 13 may be inconvenient to use. However,the resistive component of the compensation AF R,) will be usually smallcompared to AF(C,); consequently, an approximate AF given by Equation lcan be first used to calculate the thermistor-capacitor network in termsof capacitance change (AC) alone. The correct amount of AC resistiveloading (resistance R in parallel with variable capacitor 38 can then beselected to obtain the best results. The resistance 32 (R in FIG. 3serves the dual function of conventional DC bias and sets the ACresistance to the correct value to provide the correct amount ofresistive loading in parallel with the variable capacitor 38.

An example of component values for the oscillator circuit shown in FIG.3 wherein compensation is provided by making use of both the capacitiveand resistive changes of the thermistor-capacitor network is listedbelow. The circuit shown in FIG. 3 includes a capacitor 52 and resistor51 which provides the load termination.

Effective trimmer range including the load termination (33 pfI) and thecollector-to-emitter output capacitance of transistor 40 (2 pf.) isequal to 40 pf.-60 pf.

What is claimed is:

l. A temperature compensated crystal oscillator comprisa semiconductordevice having an input electrode, an output electrode and commonelectrode, connection means for applying energizing potentials betweensaid electrodes,

a frequency controlling resonant circuit including a crystal connectedin series with a series combination of a variable capacitor and a firstfixed capacitor coupled between said input and said output electrode,

regenerative feedback means comprising a pair of series connected fixedcapacitors connected in shunt to said resonant circuit,

a connection from the junction point of said series connected fixedcapacitors to said common electrode to pro vide oscillations, saidcrystal being frequency sensitive to changes in the crystal loadcapacitance, to changes in temperature and to long term crystal aging.

a network comprising a temperature variable resistance in series with acapacitance responsive to said temperature changes at a given selectedfrequency and connected in said oscillator provide a given degree ofcrystal load capacitance change over a given frequency range so as tokeep said selected frequency within a given frequency tolerance forfrequency shifts due to said temperature changes,

said variable capacitor functioning to correct the crystal frequencydrifts due to long term crystal aging and which when varied changes saiddegree of A crystal load capacitance by said network so that saidselected frequency is outside said given frequency tolerance, saidnetwork being connected across only said first fixed capacitor therebyreducing said changes brought about by the setting of said variablecapacitor in the degree of load capacitance change by said network tothereby maintain said given frequency tolerance. 2. The combination asclaimed in claim 1 wherein the values of said pair of series connectedfixed capacitors in shunt with said resonant circuit are relativelylarge compared to said series combination of said variable and saidfirst fixed capacitors making the oscillator frequency primarilydependent on the crystal in series with said series combination of saidfirst fixed and said variable capacitor.

II i UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3641 461 Dated FCbIUHIY 8 1972 lnventofls) Pawel K. Mrozek It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Data-Cover Sheet, Item [63], under "Related U.S. Application Data"correct "Continuation-in-Part" to read --Continuation--.

Column 1, line 24, correct "(C to read (C Column 3, line 30, correct"larger" to read -large-.

Column 6, line 5, correct "-0 .10 110 to read 7 I 20 c v 6 f 1+ 0 Cf CSigned and sealed this 22nd day of August 1972.

(SEAL) Attest:

EDWARD M.FLETGHER,JR. ROBERT GOTTSGHALK Attesting Officer Commissionerof Patents FORM P0-1050 (10-69) uscoMM-Dc 60378-P69 9 U.S. GOVERNMENTPRINTING OFFICE: [969 0-3ES334.

1. A temperature compensated crystal oscillator comprising: asemiconductor device having an input electrode, an output electrode andcommon electrode, connection means for applying energizing potentialsbetween said electrodes, a frequency controlling resonant circuitincluding a crystal connected in series with a series combination of avariable capacitor and a first fixed capacitor coupled between saidinput and said output electrode, regenerative feedback means comprisinga pair of series connected fixed capacitors connected in shunt to saidresonant circuit, a connection from the junction point of said seriesconnected fixed capacitors to said common electrode to provideoscillations, said crystal being frequency sensitive to changes in thecrystal load capacitance, to changes in temperature and to long termcrystal aging, a network comprising a temperature variable resistance inseries with a capacitance responsive to said temperature changes at agiven selected frequency and connected in said oscillator provide agiven degree of crystal load capacitance change over a given frequencyrange so as to keep said selected frequency within a given frequencytolerance for frequency shifts due to said temperature changes, saidvariable capacitor functioning to correct the crystal frequency driftsdue to long term crystal aging and which when varied changes said degreeof crystal load capacitance by said network so that said selectedfrequency is outside said given frequency tolerance, said network beingconnected across only said first fixed capacitor thereby reducing saidchanges brought about by the setting of said variable capacitor in thedegree of load capacitance change by said network to thereby maintainsaid given frequency tolerance.
 2. The combination as claimed in claim 1wherein the values of said pair of series connected fixed capacitors inshunt with said resonant circuit are relatively large compared to saidseries combination of said variable and said first fixed capacitorsmaking the oscillator frequency primarily dependent on the crystal inseries with said series combination of said first fixed and saidvariable capacitor.